The present invention relates to solid-state microwave devices including oscillators, amplifiers, phase shifters, and attenuators. In particular, the present invention relates to Avalanche-Transit-Time Devices.
Avalanche Transit-Time Devices provide a negative resistance which can in principle be used to operate at extremely high microwave frequencies, since the transit time which limits the frequency response is a transit time in a vertical direction, and can be therefore controlled by layer thickness. In addition, these devices are in principle capable of high power densities at extreme microwave and millimeter frequencies. However, the theoretical potential of such structures has in the past proved difficult to take advantage of, largely due to problems of external impedance matching, which have made it difficult to effectively couple power out of these devices, which are typically fabricated physically small to increase impedance levels. In particular, when they are configured as discrete devices, as is common, the individual IMPATTs (or other Avalanche Transit-Time Diode type) are physically tiny, (area being inversely proportional to frequency) and their assembly in a power-combining package will therefore typically contain significant mismatches due to assembly error, which lowers the maximum number of diodes which can be combined. It has therefore been difficult to combine enough IMPATTs to get usefully large amounts of power out at higher microwave and millimeter-wave frequencies, e.g. 94 GHz.
A further important consideration in the art of Avalanche Transit-Time Devices is that such devices are typically quite noisy. Thus, when such a device is to be used, e.g. as a local oscillator, the filtering which is required for reduction of device noise will impose significant additional loses on the already minimal output power of the device. Thus, output power for Avalanche Transit-Time Devices is generally at a premium.
It is an object of the present invention to provide an Avalanche Transit-Time Device such that the outputs of multiple devices can easily be combined for high power.
It is a further object of the present invention to provide a Avalanche Transit-Time Device having an easy and practical means for coupling the output power out.
The possibility of distributed semiconductor diode structures with negative resistance has been discussed in the prior art. The Hines paper, "High-frequency IMPATT-resistance circuit principles for Esaki diode applications", Bell System Technical Journal Volume 39, page 477 (1960), which is hereby incorporated by reference, is primarily directed to tunnel diodes (and does not mention IMPATTs), but does mention the possibility of a distributed semiconductor structure having gain. The Davydova et al. paper, "Linear Theory of an IMPATT Diode Distributed Microwave Amplifier," in Telecommunications and Radio Engineering, Part 2, Volume 27, page 112 (1972), which is hereby incorporated by references, does discuss the possibility of a distributed IMPATT. The Midford et al article, "A two-port IMPATT Diode Travelling-Wave Amplifier, which appeared at pages 1724 and 1725 of the Proceedings of the IEEE in 1968, provides a cursory description of an allegedly functional distributed-IMPATT device built in silicon. However, at the time of this article it had not yet been realized that the optimal conductivity for the contact (p+ and n+) layer in the IMPATT is not infinite. The Hambleton et al article "Design Considerations for Resonant Travelling Wave IMPATT Oscillator," International Journal of Electrics, Volume 35, pages 225-244 (1973) provided a greatly improved theoretical analysis of distributed IMPATT structure. Finally, the two Franz and Beyer articles, "The Travelling-Wave IMPATT mode," IEEE Transactions in Microwave Theory and Techniques, Volume MTT-26, page 861 (1978), and "The Travelling-Wave IMPATT mode: Part II-The Effective Wave Impedance and Equivalent Transmition," IEEE Transactions in Microwave Theory and Techniques," Volume MTT-28, pages 215-218 (1980), taught what is now the standard theoretical analysis of distributed IMPATT operation. In particular, FIG. 10 of the second Franz and Beyer article sets forth the standard modeling for a distributed IMPATT structure as a transmission line with gain. This model is followed in the discussion of the present invention. All of these references are hereby incorporated by reference.
In a distributed IMPATT structure, power is coupled out through a side contact. That is, in previously proposed distributed IMPATT structures the gain medium (the active region of the IMPATT) operates as a transmission line. The prior art has attempted to couple output power from the gain medium through an end contact, i.e. through a contact which intercepts the primary direction of energy propagation (and also to the direction of maximum elongation) of the active medium. In the present invention, a side contact extends along the whole active region in a direction which is parallel to the principal direction of propagation of the energy in the active medium. Thus, the sidewall contact plus the active region together can be considered as a single transmission line.
The present invention can be configured both as oscillators and as amplifiers. When configured as an oscillator, multiple short active regions can be sequentially coupled to a single long microstrip, which serves as the sidewall contact for each of the active regions. This very simple power combining scheme provides extremely high power at millimeter wave frequencies.
According to the present invention there is provided a microwave device comprising:
a semiconductor diode active region interposed between frontside and backside contacts, said semiconductor active region defining a negative resistance diode between said frontside and backside contacts;
said semiconductor active region and said frontside and backside contacts being elongated in a first direction;
said frontside contact being extended beyond said active region in a direction normal to said first direction, to form a transmission line having a principal direction of propagation substantially parallel to said first direction.